Process and apparatus for cryogenic separation of air with mixed gas turbine

ABSTRACT

In this process and apparatus for cryogenic separation of air, the separation column system comprises a high-pressure column, a low-pressure column and a crude argon column. A mixed gas stream produced by mixing gaseous oxygen and a gas stream from the evaporation space of the argon top condenser, is work expanded in a mixed gas turbine.

The invention concerns a process for cryogenic separation of air and arespective apparatus according to the first parts of the independentpatent claims.

Cryogenic separation of air generating gaseous and liquid products isgenerally known, e.g., from H.-W. Haring, Industrial Gases Processing,Wiley-VCH, 2006, in particular section 2.2.5, “Cryogenic Rectification”.

Cryogenic air separation units classically comprise separation columnsystems in form of two-column systems, in particular Linde doublecolumns. The may also have the form of three- or more-column systems. Inaddition to those columns for oxygen-nitrogen separation for generationnitrogen and/or oxygen in liquid and/or gaseous form, the separationcolumn system may comprise additional columns for recovering further aircomponents, in particular noble gases, or for producing particularhigh-purity oxygen and/or nitrogen products.

In the invention, a high-pressure column and a low-pressure column,which may be at least partially located above the high-pressure columnand a main condenser, are used. The process of the invention is of theenhanced-pressure type, so that the high-pressure column is not operatedat the classical pressure of about 5.3 bar (4 to 7 bar), but at a higherpressure of e.g. 8 to 14 bar, preferably 9 to 13 bar. The low-pressurecolumn is not operated at the classical pressure of about 1.3 bar (1.2to 1.5 bar), but at a higher pressure of e.g. 2 to 5 bar, preferably 2.5to 4.5 bar. Those pressures are absolute and to be measured at the topof the respective column and also used in the invention.

If an air separator produces pressurized gaseous products, they may becompressed in a gas compressor (“external compression”). Alternatively,an “internal compression” process may be used by withdrawing cryogenicliquid from the column, pressurizing (e.g. pumping) is to the desiredpressure and transferring it to the gaseous state by warming, e.g. inthe main heat exchanger.

The invention has the object to find a further improved air separationprocess, in particular for co-production of pressurized nitrogen andargon and a relatively high liquid production, e.g. a liquid production(LIN equivalent [Nm3/h]: LIN [Nm3/h]+1.07×LOX [Nm3/h]+0.9×LAR [Nm3/h])divided by pressurized GAN product amount is in a range from 0.00 to0.06. (All those amounts are molar in this application, as long asnothing to the contrary is said.)

Such object is solved by the process and the apparatus according theindependent patent claims.

The fraction withdrawn from the high-pressure column and introduced intothe low-pressure column is frequently the bottom fraction of thehigh-pressure column. At least a part of it can be directly introducedinto the low-pressure column, eventually through a subcooler, or thatcan be done indirectly by leading the high-pressure column fraction intothe argon top condenser evaporation space and separately introducing gasand remaining liquid from the argon top condenser evaporation space tothe low-pressure column.

The gaseous oxygen stream is normally withdrawn from a lower portion ofthe low-pressure column, e.g. from the very bottom of the low-pressurecolumn.

The expansion machine may be of any type, e.g. a turbine; then it may becalled a “mixed gas turbine”.

For enhancing of the rectification in the low-pressure column, anitrogen recycle coming from the low-pressure column and directly orindirectly leading into the separation column system, in particular intothe high-pressure column and/or into the low-pressure column may be usedas described in claim 2. The “recycle gas” stream is the one coming fromthe low-pressure column, being compressed in the nitrogen compressor andafterwards being cooled, but not liquefied in the main heat exchanger. Aproduct gas from the low-pressure column may or may not be lead jointlywith the recycle gas through the warming in the main heat exchanger andthe compression in the nitrogen compressor. The cooled recycle gas mayat least partially be directly in gaseous form led into thehigh-pressure column, e.g. at the top or at 3 to 11 theoretical traysbelow. An alternative is an indirect introduction into the high-pressurecolumn and/or the low-pressure column, e.g. by liquefying the recyclegas in a condenser, e.g. the main condenser and/or another columnreboiler, and then introducing at least a portion of the liquefiedrecycle gas into a column, in particular the high-pressure column and/orthe low-pressure column. In a first example, at least a portion of thecooled recycle gas is introduced into the high-pressure column via theliquefaction space of the main condenser. In another example, at least aportion of the cooled recycle gas is introduced into the low-pressurecolumn via the liquefaction space of the bottom condenser of a pureoxygen column (preferably including subcooling this liquid in a separatechannel in a subcooler and expansing the subcooled liquid in anexpansion valve). For example, a first portion of the recycle gas is ledvia a first path (e.g. directly or via the main condenser) into thehigh-pressure column and a second portion of the recycle gas is led viaa second path (e.g. through the bottom condenser of a pure oxygencolumn) into the low-pressure column.

In a first variant, cooled recycle gas may be directly introduced intothe high-pressure column, e.g. at its top. Claim 3 describes a secondvariant, where recycle gas is introduced into the main condenser,liquefied therein and then introduced as liquid onto the top of thehigh-pressure column. Both variants can be combined by introducing oneportion of the cold recycle gas into the main condenser, another portiondirectly into the column. Another portion of the recycle gas may be usedat a different place in the plant.

A pressurized pure argon product may be generated by internalcompression as lined out in claim 4. A part of the total argon productcan be produced in a liquid form and stored in the tank.

The crude argon column may have the form of a split column as describedin claim 5. There are at least two parts. In principle, there can bethree or more parts.

The separation may further comprise a pure oxygen column as described inclaim 6. The feed liquid for the pure oxygen column comes from thebottom of the crude argon column or from an intermediate point of thecrude argon column, e.g. a few theoretical trays above the bottom.

Such pure oxygen column is preferably pure oxygen column is arrangedbelow the first part of the crude argon column and within a commonvessel with the first part of the crude argon column.

The pure oxygen column preferably has a bottom reboiler as described inclaim 8, which may be heated high-pressure column gaseous nitrogenand/or by a portion of the cooled recycle gas, which is not directlygoing into the high-pressure column—see claim 9. The recycle gas ispreferably at least partially liquefied in the bottom reboiler of thepure oxygen column and then sent to the high-pressure column or to thelow-pressure column as reflux liquid.

In an operation mode, in which not the entire argon product is needed,an argon-oxygen mixture may be withdrawn from the crude argon column viaan intermediate gas outlet according to claim 10. That feature reducesthe load of the crude argon column. The argon-oxygen mixture is warmedin the main heat exchanger in order to recover its energy.

This particular embodiment is applicable to a one-part crude argoncolumn as well as to a split crude argon column. In the last case, theintermediate gas outlet may be in either part of the crude argon column.Preferably it is arranged at an intermediate height of the second part.

In the invention, it may be advantageous to use a split low-pressurecolumn as described in claim 11.

In a variant of a process, there is preferably not recycle gas and topgas of the high-pressure column (12) is withdrawn (302) as a pressurizedgaseous nitrogen product as lined out in claim 13. Alternatively oradditionally, top gas (64, 65) from the low-pressure column (13,113/213) is compressed in a nitrogen compressor and withdrawn as apressurized gaseous nitrogen product, in particular by admixing it thewarmed top gas from the high-pressure column (12). The nitrogencompressor preferably does not compress further streams, in particularno recycle gas.

The invention and further details of the invention will be illustratedin the following by exemplary embodiments, which are shown in thedrawings.

FIG. 1 a first embodiment of the invention with single-part low-pressurecolumn,

FIG. 2 a second embodiment with split low-pressure column and

FIG. 3 a third embodiment with partial withdrawal of GAN product fromtop of the high-pressure column

In the embodiment of FIG. 1 , atmospheric air (AIR) 1 flows through afilter 2 to a main air compressor 3 and is compressed therein to apressure of about 11 to 12 bar. The compressed air stream is cooled incoolers 4 and 5 and the sent to separator 6, from where liquid water(H2O) is discharged. The air from separator 6 is sent to purificationunit 7 removing water vapour, carbon dioxide and further impurities byadsorption. The purified air 8 is introduced into a main heat exchanger9. The total feed air is fully cooled until the cold end of the mainheat exchanger 9 and then introduced into the high-pressure column 12 ofa double column further comprising a low-pressure column 13 and a maincondenser 14.

The separation column system of the embodiment of FIG. 1 consists of thedouble column 12/13, a pure oxygen column 16, a methane rejection column17, a single-part crude argon column 18 and a pure argon column 19. Thepure oxygen column has a bottom reboiler 20, the crude argon column atop condenser 21 and the pure argon column a top condenser 22 and abottom reboiler 23. All these condensers and reboilers as well as themain condenser 14 are condenser-evaporators, each having a liquefactionspace and an evaporation space. An exception is the bottom reboiler 23of the pure argon column 19, which is warmed by sensible heat.

The crude liquid oxygen 24 from the bottom of the high-pressure column12 is cooled in a subcooler 25. A first portion 27 of the cooled crudeliquid oxygen 26 is partially fed through the bottom reboiler 23 of thepure argon column and then introduced into the evaporation space of thetop condenser 21 of the crude argon column 18. The remaining liquid 28is sent to the low-pressure column 13. A first part 30 of the evaporatedportion 29 is sent to the low-pressure column as well. A second part 31is taken as “the stream having a higher nitrogen content” 31 accordingto the invention and is described in detail later.

A second portion 32 of the cooled crude liquid oxygen 26 is introducedinto the evaporation space of the top condenser of the pure argon column19. Remaining liquid 33 is sent to the low-pressure column 13. Theevaporated portion 34 is mixed to the evaporated portion 29 from theevaporation space of the top condenser 21 of the crude argon column 18.Thereby is goes to the low-pressure column 13 or into the “the streamhaving a higher nitrogen content” 31.

Most 36 of the gaseous nitrogen 35 from the top of the high-pressurecolumn 12 is at least partially liquefied in the main condenser 14. Theremainder 37 in at least partially liquefied in the bottom reboiler ofthe pure oxygen column. The liquid nitrogen from the pure oxygen columnbottom reboiler is cooled in the subcooler 25. The cooled liquidnitrogen 39 is sent to the top of the low-pressure column 13.

The liquid nitrogen 40 from the main condenser 14 is partially fed backto the top of the high-pressure column 12. Another portion 42 is cooledin the subcooler 25. A first part of the cooled liquid nitrogen 43 issent to the top of the low-pressure column 13, whilst a second part 45is withdrawn as pure liquid nitrogen product (PLIN).

A gaseous argon-containing fraction, the argon transition fraction 46,from the low-pressure column 13 is introduced into the bottom of themethane rejection column 17. In the other direction, the bottom liquid47 of the methane rejection column 17 is reintroduced into thelow-pressure column 13. Such bottom liquid contain practically allmethane from fraction 46, so that the top of the methane rejectioncolumn 17 is methane-free. The top gas 48 of such column is sent to thebottom of the crude argon column 18, together with the top gas 80 fromthe pure oxygen column 16.

The bottom liquid 78 of the crude argon column 18 is lifted via pump 79.A first portion 49 enters the pure oxygen column 16 as methane-freereflux. From the bottom of the pure oxygen column 16, ultra-high purityliquid oxygen 50 is withdrawn and led into a storage tank 51. The tankliquid may be pressurized in the tank or downstream the tank by a pump(not shown). The high-pressure liquid oxygen can be warmed in the mainheat exchanger 9 and be recovered as an internally compressed ultra-highpurity gaseous oxygen product (GOXIC).

A second portion 52 of bottom liquid 78 of the crude argon column 18 isfed into the top of the methane rejection column 17.

The liquefaction space of the top condenser 21 of the crude argon column18 is a bath type condenser. At its top, a crude argon stream 58 iswithdrawn from the crude argon column 18 and introduced into the pureargon column 19. From the top of the pure argon column, a waste gas 60is withdrawn and released to the atmosphere (ATM). At the bottom, a pureargon product 59 is recovered and the sent to an internal compressionwith pump 61 and (line 62) warming in main heat exchanger 9. At the warmend of the main heat exchanger 9 (line 63), an internally compressedgaseous argon product is (GARIC) withdrawn in pressurized form.

The gaseous nitrogen fraction 64 from the top of the low-pressure column13 is partially used as a recycle gas and first pre-warmed in subcooler25. The pre-warmed gaseous nitrogen fraction 65 is sent to the cold endof the main heat exchanger 9 and fully warmed therein. The warmedgaseous nitrogen fraction 66 is compressed in a nitrogen compressor 67to a product pressure of preferably 8 to 15 bar, more preferably 9.5 to12.5 bar. The compressor 67 has an aftercooler. The compressed nitrogenfraction 68 is split into a product fraction 69, which is withdrawn aspressurized gaseous nitrogen product (PGAN) and the recycle gas 70. Thepressurized recycle gas is fully cooled again in the main heat exchanger9. The cooled recycle gas (89) is mixed with the gaseous nitrogen 35from the top of the high-pressure column 12, i.e. liquefied either inthe main condenser 14 or in the pure oxygen column bottom reboiler 20.Thereby, a portion of the recycle gas (now as liquid) enters thehigh-pressure column via line 41.

Pressurized gaseous oxygen is produced by internal compression. Liquidoxygen 84 from the bottom of the low-pressure column 13 (or from theevaporation space of the main condenser 14) is pumped in pump 85 to thedesired product pressure, fully warmed in the main heat exchanger 9 andfinally recovered via line 86 as internally compressed product (GOXIC).

The previously mentioned “the stream having a higher nitrogen content”31, which comes at least partially from the evaporation space of the topcondenser 21 of the crude argon column 18 is warmed in the subcooler 25.The warmed stream 71 is mixed with a gaseous oxygen stream 72 from thebottom of the low-pressure column 13. The mixed gas 73 is partiallywarmed in the main heat exchanger 9 to an intermediate temperature of150 to 230 K and work-expanded in a mixed-gas turbine 75, which isoperated as a generator turbine. The expanded mixed gas 76 isreintroduced into the main heat exchanger 9 and fully warmed. The warmedlow-pressure mixed gas 77/78 can be released to the atmosphere (ATM) orsent to the purification unit 7 as regeneration gas.

In the embodiment of FIG. 1 , some of the gas rising in the crude argoncolumn 18 may be withdrawn via an intermediate gas outlet 81, in orderto reduce the amount of argon product 59/62/63 and thereby reducingenergy consumption. The gas withdrawn gas 82 is fully warmed in aseparate passage of the main heat exchanger 9. The warmed gas 83 may beadmixed to the expanded mixed gas 77 and either released to theatmosphere or used as regeneration gas in the purification unit 7.

The process of FIG. 2 mainly differs from FIG. 1 by a split argon columnand a split low-pressure column. The explanations on FIG. 1 above arealso valid for the respective steps and units of FIG. 2 . Referencenumbers in FIG. 2 are partially taken from FIG. 1 in order to identifythe same or similar features and functions.

The crude argon column is split into a first part 118 and a second part218, the argon top condenser 21 being arranged on the top of the secondpart 218. A gas fraction 190 from the top of the first part 118 isintroduced into the bottom of the second part 218. At least a firstportion 193 of the bottom liquid 191 of the second part 218 isintroduced into the top of the first part 118.

The low-pressure column is split into a bottom part 113 and a top part213. Different from a one-part low-pressure column, those two parts arearranged side by side. A gaseous connection stream 195 is taken from thetop gas 194 of the bottom section and introduced into the bottom of thetop section 213. A liquid connection stream 196 is withdrawn from thebottom of the top section 213 and sent to the top of the bottom section213 via the bottom of the first part 118 of the crude argon column, line197, pump 198 and line 199. Another portion of the top gas 194 of thebottom section 113 of the low-pressure column is taken as argontransition fraction 46 and introduced into the bottom of the first part118 of the crude argon column. The bottom liquid of the first part 118(mixed with the bottom liquid 196 from the top part 213 of thelow-pressure column) is sent via line 197, pump 198 and line 199 to thetop of the bottom part 113 of the low-pressure column.

The lowermost section 117 of the first part 118 of the crude argoncolumn simultaneously acts as methane rejection column. At anintermediate height immediately above the lowermost section 117, thefirst part 118 is connected to the top of a pure oxygen column 16 by aliquid line 149 and a gas line 180.

Ultra-high purity liquid oxygen 50 from the bottom of the pure oxygencolumn 16 is pressurized in this particular embodiment in a multipletank system 200 according to U.S. Ser. No. 10/209,004 B2 and then (vialine 201) fully warmed in the main heat exchanger 9. The warm ultra-highpurity oxygen gas 202 is recovered as a final product (UHPGOX).

The liquid oxygen 84 from the bottom of the low-pressure column 113 (orfrom the evaporation space of the main condenser 14) is subcooled insubcooler 25 (not shown) and then withdrawn as a liquid oxygen product(LOX).

The cooled recycle gas 89 is fed to the liquefaction space of the maincondenser 14 (together with some of the top nitrogen 35 from thehigh-pressure column 12). There it is liquefied. A first portion 41 ofthe liquefied recycle gas is fed into the top of the high-pressurecolumn 12; a second portion 42, 44 of the liquefied recycle gas is fedinto the top of the low-pressure column 213.

Alternatively, the cooled recycle gas 89 could be split into a firstportion to the main condenser and a second portion, which is introducedinto the liquefaction space of the bottom reboiler of the pure oxygencolumn 16. In another alternative, the recycle gas is completely fed tothe liquefaction space of the bottom reboiler of the pure oxygen column16, if necessary supplemented by some gaseous nitrogen 35 from the topof the high-pressure column 12.

FIG. 3 is in many parts similar or identical to FIG. 2 , but deviates intwo main aspects:

-   -   Gaseous nitrogen is from the top of the high-pressure column and        withdrawn as a pressurized gaseous nitrogen product (UHPGAN) via        lines 300, 301 and 302.    -   There is no recycle gas. All of the top gaseous nitrogen        64/65/66/369 from the low-pressure column 213 is withdrawn as a        pressurized gaseous nitrogen product (UHPGAN) downstream the        nitrogen compressor 67 by admixing it to the nitrogen from the        high-pressure column 12.

The invention in general can be applied as well to systems without amethane rejection column and/or without a pure oxygen column.

1. Process for cryogenic separation of air in a separation column systemcomprising a high-pressure column, a low-pressure column, a maincondenser, which is a condenser-evaporator having a liquefaction spaceand an evaporation space and brings high-pressure column top andlow-pressure column bottom in heat-exchange relationship, and a crudeargon column having an argon top condenser, which is acondenser-evaporator having a liquefaction space and an evaporationspace, comprising compressing a total feed air stream, cooling thecompressed feed air in a main heat exchanger, introducing at least aportion of the feed air into the high-pressure column, introducing atleast one fraction from the high-pressure column directly or indirectlyto the low-pressure column, introducing an argon transition fractionfrom the low-pressure column to the crude argon column, introducing aliquid cooling fraction from the high-pressure column into theevaporation space of the argon top condenser, withdrawing a gaseousoxygen stream from the low-pressure column, mixing the gaseous oxygenstream with another gas stream having a higher nitrogen content than thegaseous oxygen stream to form a mixed gas stream, warming the mixed gasstream in the main heat exchanger, work expanding the warmed mixed gasstream in an expansion machine and completely warming the expanded mixedgas stream in the main heat exchanger, wherein the above stream having ahigher nitrogen content is withdrawn from the evaporation space of theargon top condenser.
 2. Process according to claim 1, wherein a gaseousnitrogen fraction from the low-pressure column is used as a recycle gas,the recycle gas is warmed in the main heat exchanger, the warmed recyclegas is compressed in a nitrogen compressor, the compressed recycle gasis cooled in the main heat exchanger, and withdrawn from the main heatexchanger in gaseous form and at least a first portion of the cooledrecycle gas is introduced either in gaseous form or in liquefied forminto the separation column system, in particular into the high-pressurecolumn and/or the low-pressure column.
 3. Process according to claim 2,wherein at least a portion of the cooled recycle gas is introduced intothe high-pressure column via the liquefaction space of the maincondenser.
 4. Process according to claim 1, wherein the separationcolumn system further comprises a pure argon column, a crude argonstream is withdrawn from the crude argon column or the argon topcondenser, the crude argon stream is introduced into the pure argoncolumn, a liquid pure argon stream is withdrawn from the pure argoncolumn, the liquid pure argon stream is pressurized in liquid state, thepressurized pure argon stream is warmed in the main heat exchanger andfinally recovered as a pressurized argon product.
 5. Process accordingto claim 1, wherein the crude argon column is split into a first partand a second part, the argon top condenser being arranged on the top ofthe second part, whereby a gas fraction from the top of the first partis introduced into the bottom of the second part and at least a firstportion of the bottom liquid of the second part is introduced into thetop of the first part.
 6. Process according to claim 5, wherein theseparation column system further comprises a pure oxygen column, aliquid fraction from the crude argon column is introduced into the topof the pure oxygen column and a liquid pure oxygen fraction is withdrawnfrom the bottom of the pure oxygen column.
 7. Process according to claim6, wherein the pure oxygen column is arranged immediately below amethane rejection column having just a single bottom/top plate betweeneach other.
 8. Process according to claim 6, wherein the pure oxygencolumn has a bottom reboiler, which is a condenser-evaporator having aliquefaction space and an evaporation space.
 9. Process according toclaim 2, wherein a second portion of the cooled recycle gas isintroduced into the liquefaction space of the pure oxygen column bottomreboiler.
 10. Process according to claim 1, wherein, at leasttemporarily, an argon-oxygen mixture is withdrawn from the crude argoncolumn via an intermediate gas outlet and the argon-oxygen mixture iswarmed in the main heat exchanger.
 11. Process according to claim 1,wherein the low-pressure column is split into a bottom part and a toppart, a gaseous connection stream is withdrawn from the top of thebottom section the gaseous connection stream is introduced into thebottom of the top section, a liquid connection stream is withdrawn fromthe bottom of the top section and the liquid connection stream isintroduced into the top of the bottom section.
 12. Process according toclaim 1, wherein a portion of the top gas of the high-pressure column iswarmed in the main heat exchanger and the warmed gas is withdrawn as apressurized gaseous nitrogen product.
 13. Process according to claim 1,characterized in that a top gas from the low-pressure column is warmedin the main heat exchanger, the warmed gas is compressed in a nitrogencompressor and the compressed gas is withdrawn as a pressurized gaseousnitrogen product, in particular by admixing it the warmed top gas fromthe high-pressure column.
 14. Process according to claim 1,characterized in that the cooled recycle gas is introduced into thehigh-pressure column in gaseous form.
 15. Apparatus for cryogenicseparation of air comprising a separation column system comprising ahigh-pressure column, a low-pressure column, a main condenser, which isa condenser-evaporator having a liquefaction space and an evaporationspace and is configured to bring high-pressure column top andlow-pressure column bottom in heat-exchange relationship, and a crudeargon column having an argon top condenser, which is acondenser-evaporator having a liquefaction space and an evaporationspace, and further comprising a main air compressor for compressing atotal feed air stream, a main heat exchanger for cooling the compressedfeed air, a main air compressor for compressing a total feed air stream,a main heat exchanger for cooling the compressed feed air, means forintroducing at least a portion of the feed air into the high-pressurecolumn, means introducing at least one fraction from the high-pressurecolumn directly or indirectly to the low-pressure column, an argontransition line for introducing an argon transition fraction from thelow-pressure column to the crude argon column, means for introducing aliquid cooling fraction from the high-pressure column into theevaporation space of the argon top condenser, means for withdrawing agaseous oxygen stream from the low-pressure column, means for mixing thegaseous oxygen stream with another gas stream having a higher nitrogencontent than the gaseous oxygen stream to form a mixed gas stream, meansfor introducing the mixed gas stream into the main heat exchanger forwarming, an expansion machine for work expanding the warmed mixed gasstream and means for completely warming the expanded mixed gas stream inthe main heat exchanger, wherein means for mixing the gaseous oxygenstream with another gas stream having a higher nitrogen content areconnected to the evaporation space of the argon top condenser.